Relative to an American in the money (6)
I believe the answer is:
cousin
'relative' is the definition.
(I know that relative can be written as cousin)
'an american in the money' is the wordplay.
'an american' becomes 'us' (abbreviation for United States).
'in the' indicates putting letters inside.
'money' becomes 'coin' (coin is a type of money).
'us' going into 'coin' is 'COUSIN'.
'to' acts as a link.
(Other definitions for cousin that I've seen before include "Child of a parent's sibling, say" , "Aunt's/uncle's child" , "Close relative" , "person once removed?" , "Family relative" .)